1. Field of the Invention
The present invention relates to a valve operation device of an internal combustion engine, and particularly, to a valve operation device having a variable valve characteristic mechanism which controls valve operation characteristics including an opening timing of an engine valve having an intake valve or an exhaust valve.
2. Description of the Related Art
A variable valve operation device disclosed in U.S. Pat. No. 6,019,076, for example, is related to such the valve operation device. The variable valve operation device has a camshaft interlockingly rotating around a crankshaft, a rocking cam rockably supported on a camshaft to open and close an intake valve or an exhaust valve, a control member on which a rocker lever being rocked by a rotational cam integrally rotating with the camshaft to rock the rocking cam is pivotally supported, and an actuator rocking the control member rockably supported on the camshaft. And, the actuator rocks the rocking cam around the camshaft via the control member, and thus an opening timing and a closing timing of the intake valve or the exhaust valve and the most lift amount are controlled.
Generally, a cam swelled portion of a valve operation cam which opens and closes an engine valve has a damping portion which has a low lift velocity, that is, a ratio of a change in height of the cam swelled portion with respect to a change in rotational angle of the camshaft and which includes a constant velocity portion, in order to decrease the slapping sound caused by a crash of a cam or a cam follower into the engine valve due to a valve clearance when the engine valve starts to open and to decrease the slapping sound which is caused when the engine valve is seated in a valve sheet 24 for closing.
In addition, when the damping portion is provided on the rocking cam (corresponding to a valve operation cam) of the conventional art disclosed in U.S. Pat. No. 6,019,076, a rocking angular velocity is responsible for the slapping sound in the damping portion of the rocking cam. Hereinafter, in relation to the conventional art, this will be described for the opening timing of the engine valve with reference to FIGS. 13 and 15. A position with respect to a rotational angle of the camshaft changes according to rocking positions G1 and G2 of the control member. Here, it is assumed that the opening timing of the engine valve is more advanced in the rocking position G1 than in the rocking position G2. As for the cam swelled portion of the rotational cam (corresponding to a driving cam), in rotational positions α1 and α2 corresponding to the opening timing (when the valve clearance is 0) in the damping portion of the rocking cam when the control member is disposed in the rocking positions G1 and G2, as shown in FIG. 13, a lift velocity of the cam swelled portion of the rotational cam (here, the lift velocity corresponds to the rocking angular velocity of the rocking cam which is rocked by the rotational cam via the rocker lever) has a positive acceleration. If this causes the lift velocity to increase according to the rotation of the camshaft, in the rocking position G1, the rocking cam rocks at the rocking angular velocity based on the lift velocity of the rotational cam in the rocking position G1. Accordingly, even when the slapping sound when the engine valve starts to open is set to be decreased based on the valve clearance, in the rocking position G2, the lift velocity of the rotational cam becomes large than that in the rocking position G1, and thus the rocking angular velocity of the rocking cam becomes also large than that in the rocking position G1. For this reason, in the rocking position G2, there may be a case in which a damping function of the damping portion is not fully performed in the rocking position G2 and the slapping sound is caused by the valve clearance. Further, a similar phenomenon may be caused in the closing timing of the engine valve, and thus there may be a case in which the slapping sound is caused when the engine valve is seated in the valve sheet.